Exhaust-gas analyzer probes are used, for example, in the form of lambda oxygen sensors in the exhaust passage of internal combustion engines to determine the composition of an air-fuel mixture fed to the internal combustion engine. According to statutory regulations, the exhaust-gas analyzer probes must be checked during operation for correct functioning, what is referred to as “measuring capability.” To that end, a gaseous mixture of known composition may be fed to the exhaust-gas analyzer probe, and the output signal of the exhaust-gas analyzer probe may be evaluated. When checking a lambda oxygen sensor for correct functioning in the case of a “rich” air-fuel ratio with a lambda value <1, the checking of what is referred to as the “rich gas measuring capability,” such a mixture would thus have to be supplied to the lambda oxygen sensor. Particularly for diesel engines which are operated in the range of a lean exhaust gas during normal operation, operation with a rich mixture for a diagnosis leads to increased emission of pollutants as well as additional fuel consumption, and is therefore preferably avoided. Other possibilities for making a rich gas composition available at the lambda oxygen sensor without intervening in the engine operation are not feasible for multiple reasons.
However, a reliable diagnosis in another way is also fraught with disadvantages. The pump current of the lambda oxygen sensor is used to determine the lambda value. The pump current is positive in the case of lean exhaust gas and negative in the case of rich exhaust gas. The transport processes within the lambda oxygen sensor for lean exhaust gas and rich exhaust gas differ considerably in terms of the direction of current flow, electrode reaction and gases at the electrodes. For a realistic diagnosis, these conditions must therefore be made available, in so far as necessary for the diagnosis.
Exhaust-gas analyzer probes may be realized as broadband lambda sensors, lambda voltage-jump sensors, two-cell broadband lambda sensors or single-cell limit-current sensors, but also, for example, as NOx-sensors.
German Patent Application No. DE 10 2011 005 490 A1 describes a method for operating a sensor element to detect at least one property of a gas in a measuring-gas compartment, the sensor element including at least one pump cell having at least two electrodes and at least one solid electrolyte connecting the electrodes, at least one first electrode of the pump cell being able to be acted upon with gas from the measuring-gas compartment, at least one second electrode of the pump cell being connected to at least one reference channel, the method being used to check whether a pump current through the pump cell is limited by an impingement of gas on the first electrode, or whether the pump current through the reference channel is limited. The description alludes to the possibility of a pump-voltage reversal for diagnostic purposes. However, the diagnosis of the capability of an exhaust-gas analyzer probe to measure rich gas is not discussed in the document.
German Patent Application No. DE 10 2010 040 817 A1 describes a method for the adjustment of a sensor element to detect at least one property of a gas in a measuring-gas compartment, especially to determine one constituent of a gas component, the sensor element having at least two cells, each having at least two electrodes and at least one solid electrolyte connecting the electrodes, at least one first electrode being part of both cells, the first electrode being able to be acted upon via at least one diffusion barrier with gas from the measuring-gas compartment, the cells including at least one first cell and at least one second cell, at least one gas component of the gas pumped from the first electrode through a second cell being at least partially returnable via the diffusion barrier to the first electrode, a first pump current through the first cell being measured, a second pump current through the second cell being measured, and at least one property of the diffusion barrier being inferred from the first pump current and the second pump current. The document describes that the characteristic curve of a broadband lambda sensor may be adjusted during or after production or even during field operation, e.g., in an overrun condition of a diesel engine. In doing so, suitable diagnostic processes and/or regeneration processes may be carried out, e.g., with the aid of a pump-current reversal and/or pumping up of the electrode cavity. A method for diagnosing the capability of the lambda oxygen sensor to measure rich gas is not described.
German Patent Application DE 10 2009 060 172 A1 describes a method for diagnosing a dynamic response of an exhaust-gas sensor, by which a property of an exhaust-gas flow is able to be characterized, the exhaust-gas sensor having a measuring control loop, and the property of the exhaust-gas flow being able to be characterized with the aid of an actuating signal of the measuring control loop, the method including:                producing a change in the actuating signal,        ascertaining a reaction of a measuring signal of the measuring control loop to the change, and        judging the dynamic response of the exhaust-gas sensor with the aid of the reaction.        
The document alludes to the possibility that instead of a mixture composition of the exhaust-gas mass flow, by switching off and/or switching over the regulator or its actuating signal, which may occur in an integrated circuit of the regulator or an engine control unit of a combustion engine producing the exhaust-gas flow, for example, the oxygen content in the reference cell may be changed. In this instance, the possibility for diagnosing the capability of the exhaust-gas sensor to measure rich gas for engines operated with lean combustion is not described.